The overall objective of this study is to assess constellations of two pairs of satellites for the retrieval of time-variable gravity field information for best quality scientific monitoring of mass distribution and transport in the Earth system. Before embarking on this study the geodetic community and space agencies have come to realize that weaknesses in current single-pair gravimetric satellite missions such as GRACE or GRACE-FO (to be launched in 2017) would significantly be alleviated by having a second GRACE-type pair in space simultaneously. In particular the so-called "Bender-constellation" with one polar pair and one pair on an inclined orbit, has tentatively been investigated in the geodetic literature.